Lactic acid production, separation and/or recovery process

ABSTRACT

A process for the production and isolation of lactic acid is provided. A lactate feed solution, preferably obtained from a fermentation broth is combined with and extracted by a water-immiscible base in the presence of an acidifying agent. Lactic acid is recovered from the resulting organic phase. Recovered carbonate or bicarbonate from the aqueous phase can be recycled to the fermentor and regenerated extractant can be recycled for use in the extraction.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation-in-part of applicationSer. No. 08/207,773 filed Mar. 8, 1994. application Ser. No. 08/207,773was a continuation-in-part of application Ser. No. 08/084,810 filed Jun.29, 1993. The complete disclosures of Ser. Nos. 08/084,810 and08/207,773 are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the production,separation and/or recovery of lactic acid and more particularly to theproduction, separation and recovery of lactic acid via a fermentationprocess and the separation and/or recovery of lactic acid from a lactatefeed solution such as is obtained from a fermentation broth or othersources.

[0004] 2. Description of the Prior Art

[0005] Lactic acid has long been used as a food additive and in variouschemical and pharmaceutical applications. More recently, lactic acid hasbeen used in the making of biodegradable polymers both as a replacementfor present plastic materials as well as various new uses wherebiodegradability is needed or desired. Accordingly, there is an everincreasing demand for lactic acid. The present invention aims at meetingthis demand by providing an efficient and environmentally friendlyprocess for producing lactic acid, that can, if desired, be employed toavoid the consumption of bases and acids; and, that can be used, ifdesired, to substantially reduce, if not eliminate, the formation ofwaste or byproduct salts.

[0006] Production of lactic acid is commonly carried out by fermentationof a strain of the bacterial genus Lactobacillus and more particularlyby the species Lactobacillus delbrueckii or Lactobacillus acidophilus asexamples. In general, the production of lactic acid by fermentation in afermentation broth is well known. The fermentation substrate consists ofcarbohydrates together with suitable mineral and proteinaceousnutrients. Because the lactic acid producing microorganisms areinhibited in a strongly acidic environment, the pH of the fermentationbroth must be kept above 4.5, and preferably within the range of about5.0 to 7.0, more preferably within the range of about 5.5 to 6.5, andmost preferably within the range of about 6.0 to 6.5. To maintain thispH level, suitable water-soluble basic substances or agents that arenon-toxic to the acid producing microorganism, such as alkali metalhydroxides, carbonates or bicarbonates or alkaline earth metalhydroxides or carbonates, are commonly added to the fermentation brothto neutralize the acid being produced. This results in the formation ofa lactate solution rather than the desired lactic acid product. Suchlactate solution contains the lactate anion and the corresponding cationof the substance used to neutralize the fermentation broth.

[0007] Various methods have been proposed for the recovery of lacticacid from a fermentation broth. Where the fermentation is carried out inthe presence of calcium carbonate, it is possible to recover the lacticacid by acidification with sulfuric acid. This results in theprecipitation of calcium sulfate, while free lactic acid remains in themother liquor. If desired, the mother liquor may be concentrated to upto about 90% by weight lactic acid. Subsequently, lactic acid may beextracted from the mother liquor with a suitable organic extractant, toyield an extract which is later back-extracted with water, or the acidmay be adsorbed on a suitable adsorbent and later desorbed. Theresulting aqueous lactic acid solution may then be concentrated. Thismethod has the disadvantage, however, that it irreversibly consumescalcium carbonate and sulfuric acid and leaves, as waste, largequantities of calcium sulfate, which can give rise to disposal problems.

[0008] U.S. Pat. No. 5,132,456 (King et al.) describes a process forrecovering carboxylic acid from a carboxylic acid-containing aqueousfeed stream having a pH close to or above the pK_(a) level of the acid.In accordance with that process the recovery involves what may bedescribed as a cascade type acid withdrawal operation in which thebasicity of the extractant is increased stepwise. In a first stage ofthe process, the feed stream is contacted with an adsorbent such as astrongly basic extractant or a solid anion exchanger. In a second stagethe acid-loaded adsorbent is contacted with an aqueous solution ofammonia or a low molecular weight trialkyl amine having a strongeraffinity to the carboxylic acid that is being recovered than theadsorber used in the first stage. In this way the aqueous solution of awater-soluble carboxylic acid ammonium salt is formed. This is thensubjected to heat treatment, whereby the salt is decomposed to yieldback the trialkyl amine or ammonia and free carboxylic acid. Applyingthis process to lactic acid involves the formation of salts of lacticacid with strong bases having a pK_(a) value of about 9-11; i.e. theconjugate acid of the base has a pK_(a) of 9-11. Thus, the decompositionof these salts into free lactic acid is energy intensive. Examples 12-14of the '456 patent mention the use of Alamine 336 (tricaprylylamine) forthe extraction of, among others, lactic acid from an aqueous solution,but no yields are mentioned. Upon the extraction of even smallquantities of lactic acid from a fermentation broth the pH of the brothrises rapidly to above 7. As shown in FIGS. 3 and 4 of the '456 patent,the uptake of carboxylic acids from aqueous solutions drops rapidly withan increase of the pH. It is, therefore, inherent in these examples thatthe lactic acid uptake, if any, is negligible. It is further noted thatupon heat treatment and concentration of an ammonium lactate,crystalline lactic acid does not precipitate and instead the viscosityof the solutions increases steadily as a result of self-association ofthe acid. It is thus evident that the process of U.S. Pat. No. 5,132,456is unsuitable for the recovery of lactic acid from a fermentation broth.

[0009] U.S. Pat. Nos. 4,444,881 and 4,405,717 (Urbas) describe a processfor the recovery of an organic acid from a diluted aqueous solution ofits calcium salt by adding a water-soluble trialkyl amine carbonate tothe solution to form on the one hand a water soluble trialkyl ammoniumsalt of the acid, which salt remains in solution, and on the other handcalcium carbonate which precipitates. After removal of the calciumcarbonate the remaining mother liquor is heated for the separaterecovery of the amine and the product acid. The decomposition of thetrialkylammonium salts of this reference into free acids is energyintensive.

[0010] U.S. Pat. No. 4,282,323 (Yates) describes a process for obtaininglower carboxylic acids from a salt solution of such carboxylic acid asobtained from fermentation. The process appears to be applicable to arestricted number of lower aliphatic and aromatic monocarboxylic acidsand is specifically described only in relation to acetic acid. Inaccordance with that process, the aqueous solution of a carboxylic acidsalt is concentrated in the presence of a liquid polar organic solventserving as extractant, with pressurized carbon dioxide, to convert atleast part of the salt to the corresponding free acid which is taken upby the organic phase, from which it is subsequently recovered. It isinherent in the use of a polar organic extractant that the bulk of thecarboxylic acid remains in the neutral to basic aqueous phase, andindeed the recovery rates reported in U.S. Pat. No. 4,282,323 are low,ranging between 4.8% and 18% of the acid initially present.

[0011] U.S. Pat. No. 4,275,234 (Baniel et al) is directed to a method ofrecovering various acids in their free form from aqueous solutions.Thus, the process of Baniel is not applicable to a lactate solution ofthe type commonly obtained from a fermentation process or from othersources. The essence of the Baniel et al. U.S. Pat. No. 4,275,234 is thediscovery that efficient back-extraction can be achieved by performingthe back-extraction at a temperature higher than that of the primaryextraction.

[0012] R. Bar and J. L. Geiner, Biotechnology Progress, 3, 109 (1987)studied the feasibility of extracting lactic acid from aqueous solutionby means of a long-chain trialkyl amine of low basicity, such astridodecylamine, using various tridodecylamine solutions in n-dodecanol.

Review of the Disclosure of U.S. Ser. No. 08/207,773

[0013] In accordance with the disclosure of U.S. Ser. No. 08/207,773, itwas reported that it is possible to separate and recover lactic acidfrom a lactate solution at a pH in the range of 4 to 14 in a nearlyquantitative fashion, with a desirable process. More specifically, thepreferred lactic acid separation and recovery process reported includesan extraction hereinafter (sometimes referred to as the primary orforward extraction) in the presence of a water-immiscible, long-chaintrialkyl amine and carbon dioxide. The lactate solution could beobtained from a fermentation broth or from hydrolyzed polylactide viapolylactide recycling or recovery, among possible others.

[0014] In the parent disclosure, the invention is described as providinga process for the separation and/or recovery of lactic acid from alactate solution formed by fermentation in the presence of a basicsubstance such as one selected from the group of alkali metal, alkalineearth metal or ammonium hydroxides, carbonates or bicarbonates. Theprocess steps comprise obtaining a lactate feed solution from afermentation broth or another source and combining such feed solutionwith an extractant. The particular preferred extractants disclosed aretrialkyl amines, with the extraction being in the presence of carbondioxide and with the trialkyl amine being water-immiscible and having atotal of at least 18 carbon atoms. The term “combining” is explained inthe parent as meaning a mixing or contacting of the lactate solution(aqueous phase) and the amine (organic phase) so that extraction canoccur. It was recited that preferably the lactate feed solution isformed by filtering a fermentation broth to remove biomass and othersolids and that the combining of the lactate solution and extractantspreferably occurs in the presence of carbon dioxide at a partialpressure of at least about 50 psig.

[0015] The above extraction in accordance with the parent disclosureresults in the formation of a lactic acid rich organic phase and anaqueous or aqueous-slurry phase. Each of these two phases, in accordancewith preferred further aspects of the parent disclosure, is furtherprocessed: the processing being recovery of lactic acid from the organicphase and recovery of carbonate or bicarbonate from the aqueous phase.As explained in the parent, preferably the recovered carbonate orbicarbonate is recycled to the fermentor. The organic phase from whichthe lactic acid been recovered can be recycled for use in the primaryextraction. If applied in the preferred manners described, this wouldresult in a process in which the consumption of acids and bases isavoided and in which the generation of waste salts and other by-productsis substantially reduced, if not eliminated.

[0016] In a preferred process of the parent disclosure, a countercurrentliquid-liquid extractor or extraction unit is used. During steady stateoperation, the lactate feed solution and extractant are loaded into theextractor and operated in the presence of pressurized carbon dioxide.The optimum operational pressure was described as not being critical,provided sufficient carbon dioxide is present for the primary extractionto occur. It was stated, however, that preferably the partial pressureof carbon dioxide is maintained at 50 psig or greater. It was describedthat upon leaving the extractor, the organic phase may be subjected todecompression. This would result in a release of the pressurized carbondioxide which could, if desired, be recovered for reuse in the process.

[0017] The long-chain trialkyl amines described in the parent as usefuland preferred are those in which the amines and the amine lactate saltsare immiscible with water and have a total of at least 18 carbon atoms,and preferably have from 24 to 42 carbon atoms. Typical examples of suchamines provided in the parent are trihexylamine, trioctylamine,triisooctylamine, tricaprylylamine and tridodecylamine. The parentdisclosure states that the term “amine salt” or “amine lactate salt”refers to the species formed when lactic acid or lactate is extractedinto the amine extractant phase, although the exact nature of thisspecies is not known.

[0018] In the parent, the extraction process is described as beingperformed batchwise or continuously, but that dramatically improvedseparation and ultimate recovery can be achieved with a continuousprocess and in particular a countercurrent extraction process.

[0019] In the parent it is stated that solvents of the trialkyl aminesmay also be used, if desired, as part of the extractant. It is believedthat these may be used for the purpose of diluting certain relativelyviscous trialkyl amines, enhancing the extraction, and/or stabilizingand maintaining the organic phase in a single phase substantiallyimmiscible with water. Any compatible organic solvent capable ofdissolving the amine and the amine lactate salt would be suitable,provided it is also inert to chemical reaction both with the long-chaintrialkyl amines utilized and to the amine lactate salt and lactic acid.In the parent, it was stated that the term “compatible” means misciblewith, soluble in and chemically inert. The usefulness of solvents forthese purposes is well known in the art. Specific examples, however,were described in the parent as liquid hydrocarbons such as kerosene ormineral oils, alkanols such as isopropanol, n-butanol and n-octanol andvarious ketones such as methyl-isobutyl ketone (MiBK) and nonanone,among others. In the parent it was stated that two or more differentsolvents may be used, for example a hydrocarbon and an alkanol.

[0020] According to the parent, the organic phase resulting from theprimary or forward extraction is subjected to a separation process suchas further extraction, vaporization or the like to recover the lacticacid. Also, it was stated that preferably the organic phase is subjectedto back-extraction with water to recover the lactic acid in an aqueousphase. Where the initial extracting medium also contains an alkanol orketone as a solvent, it was stated that the back-extraction may bepreceded by removal of the solvent through azeotropic steam distillationor other techniques. It was also stated that the portion of the organicphase remaining after separation of the lactic acid and, whereapplicable, the separately recovered alkanol or ketone, can be recycledfor use in the primary extraction. The aqueous lactic acid solutionresulting from the back-extraction can be removed as product and can beconcentrated, if desired.

[0021] In a preferred embodiment of the process described in the parent,carbonate or bicarbonate is present in the aqueous phase either insolution or as a solid suspension, predominantly in the form of analkali metal, alkaline earth metal or ammonium carbonate or bicarbonate,depending on the cation present in the lactate solution. This aqueousphase is preferably a suspension of sodium bicarbonate crystals and issubjected to solid-liquid separation followed by conversion of thebicarbonate into sodium carbonate by heat treatment or other techniquesknown in the art. Carbon dioxide liberated during this conversion may betrapped and recycled for use in the primary extraction. The solid-liquidseparation also yields an aqueous raffinate, substantially depleted oflactate, which is withdrawn and may be used as a constituent of animalfeed.

SUMMARY

[0022] In this section, a general discussion of the principles presentedin the parent disclosure Ser. No. 08/207,773 and its parent Ser. No.08/084,810, is provided. This section includes some additional commenton those principles, beyond the specific language of the '773 and '810applications.

[0023] In the Background section of the parent disclosure, a techniquefor lactic acid recovery involving acidification of a lactate solutionincluding calcium lactate, by sulfuric acid, was described. In general,it was stated that after the fermentation broth was acidified withsulfuric acid, using this prior art technique, and the mother liquor isconcentrated, the lactic acid is extracted with an organic extractant.It was generally shown that next the organic phase (water-immisciblephase) can be back-extracted with water (or the acid is adsorbed on asuitable absorbent) for isolation and recovery of the lactic acid. Thismethod is a general method involving the acidification of the broth withstrong acids. A strong acid can substantially lower the pH of thelactate solution, causing formation of undissociated lactic acid (i.e.no lactate but essentially only lactic acid) and a salt of the strongacid. In the technique, an extractant is then used to extract theundissociated acid. A principal problem with this approach is thegeneration of large amounts of the salt of the strong acid used in theacidification process. In addition, the salt of the strong acid istypically too weak a base to be useful as a base in the fermentationbroth to provide lactate salt.

[0024] Also in the Background of the parent, the Yates '323 referencewas discussed. It is noted that one of the carboxylic acids isolatedduring that process, is acetic acid. Acetic acid is a relatively weakorganic acid. If a stronger organic acid were used in the process, itwould typically be even more dissociated at a given or operating pH forthe extraction, than was the acetic acid, unless a very basic operatingpH were involved, i.e. one that ensured essentially all of the acid weredissociated. If the Yates' process (including extraction with a liquidpolar organic solvent as the extractant) were used, one would expecteven lower recovery of a stronger organic acid than Yates achieved foracetic acid, under the conditions of the process, since the processconcerns recovery of the acid, not the anion or base form. The recoveryrates reported in Yates '323 for acetic acid (4.8%-18%) were alreadyrelatively low. Thus, the Yates' approach would not be expected to bevery fruitful, if an acid substantially stronger than acetic acid, forexample lactic acid, were involved.

[0025] The techniques developed and reported in the parent disclosures(i.e. Ser. Nos. 08/207,773 and 08/084,810) concern and take advantage ofthe fact that lactic acid is moderate acid, not an extremely strong oneand not an extremely weak one. For purposes of the description,generally a “strong acid” will be considered to be any acid with apK_(a) of 1.0 or less. A “moderate acid” would be one having a pK_(a)greater than 1.0 and no greater than about 4.0. “Weak acids” would bethose with a pK_(a) greater than about 4.0. For purposes of classifyingan acid according to this description, the pK_(a) should generally berounded to the nearest 0.1. In general, lactic acid, with a pK_(a) ofabout 3.9, then, will be considered a moderate acid for purposes of thisdiscussion. The pK_(a) values in this paragraph refer to values at 25°C.; i.e. room temperature.

[0026] In the parent disclosures, reference was made to conductingextractions with immiscible amines such as long chain trialkyl amines.These are typically moderate bases. In general a “weak” base will beconsidered herein to be a base with a pH of half neutralization of lessthan 2.5; a moderate base will be considered to be a base with a pH ofhalf neutralization of between 2.5 and 7.0; and, a strong base will beconsidered to be a base with a pH of half neutralization of greater than7.0. The term “pH of half neutralization” is a measure of apparentbasicity of a water-immiscible base, as defined in Grinstead, R. R. etal., J. Phys. Chem., Vol. 72 #5, p. 1630 (1968), incorporated herein byreference.

[0027] In general, the invention concerns a process for recovery oflactic acid. The process includes the steps of forming a multi-phasesystem including at least a first aqueous phase and a second organic orwater-immiscible phase. The first aqueous phase is provided in a formhaving a pH of 4 to 14 and including lactate salt in solution. Theorganic or water-immiscible phase preferably includes an extractantcapable of forming a water-immiscible lactate salt, with alactate-containing component.

[0028] The process includes a step of extracting a first aqueous phasewith a water-immiscible phase by forming a water-immiscible lactate saltwith the extractant. The step of forming the lactate salt generallyincludes a step of acidifying at least one of the first aqueous phaseand the water-immiscible phase without providing the first aqueous phasewith a pH below 4. The process further includes a step of separating theresulting water-immiscible phase from a resulting aqueous phase, afterthe step of extracting; and, obtaining or generating lactic acid fromthe lactate salt of the extractant found in the water-immiscible phase.This later step is typically conducted through a form ofback-extracting. The step of acidifying is preferably conducted byadding an acid to either or both of the first aqueous phase and thesecond water-immiscible phase. It may be conducted either before thesetwo phases are brought together, or after.

[0029] Herein reference will, in some instances, be made to“lactate-containing component” in the aqueous phase. This term isintended to refer to both the anion form, lactate anion; and, the acidform, lactic acid, together. Preferred processes result in extraction ofat least 90% of the lactate-containing component in the first aqueousphase, and generally and preferably are conducted until at least 95% ofthis material is extracted into the water-immiscible phase (as lacticacid). Indeed, preferably the process is conducted such that ultimatelysuch amounts (i.e. at least 90%, and typically 95% or greater) are foundin the final yield after the step of lactic acid recovery from thewater-immiscible phase.

[0030] In general, the lactic acid separation and recovery process ofthe invention concerns a step of extracting the lactate-containingcomponent with a water-immiscible weak or moderate base. However, it ispreferred that the lactate-containing component removed be removed afterthe acidifying by addition of a moderate to weak acid. The particularpreferred acid disclosed in the parent, is gaseous CO₂, a weak acidwhich will generate carbonic acid in the solution. The carbonic acid,being a weak acid, will partly dissociate in the solution. Preferablythe CO₂ is added to the water-immiscible phase, before the multi-phasesystem is formed.

[0031] The use of the weak or moderate base for conduct of theextraction is important, since it facilitates later recovery of thelactic acid from the organic phase or layer. In particular, especiallyif a back-extraction with an aqueous phase or layer is used for finalisolation of the lactic acid, the weak or moderate base will not holdthe lactic acid sufficiently strongly to resist the partitioning of thelactic acid back into the aqueous phase. Preferably the back-extractionis not conducted with a weak base alone. Preferably it is conducted witha moderate base, or a mixture of weak base and moderate base.

[0032] The use of a relatively weak acid to acidify, prior to extractionor during extraction, is also important. The relatively weak acid willform a salt in the aqueous phase which is a relatively strong base thatcan be used directly, or after mild conversion (for example from abicarbonate to a carbonate), as a base in the fermentation broth. Theuse of a moderate acid is less preferable because in general the saltformed will be too weak a base to be readily useful as a base in thefermentation broth.

[0033] In general, the weak or moderate base, used for the extraction,will be considered immiscible if, under the extraction conditions, it issufficiently insoluble in the aqueous phase or layer such that itspresence will be no greater than about 1000 ppm, and preferred ones willhave a presence of no greater than about 200 ppm. Typically, bases willbe chosen that have a solubility of 100 ppm or less. The weak ormoderate base could be presented in the form of a solid, and thus becompletely immiscible in water.

[0034] Also, preferably the weak or moderate base is also immiscible inthe water used in the later back extraction, if one is conducted. A basewill be considered immiscible in the aqueous phase of the backextraction if the above-stated ppm limits for water-immiscibility duringthe first extraction are not exceeded.

[0035] From the above it will be apparent that the process steps ofpreferred lactic acid recovery processes, described in the parentapplications, concern the following steps:

[0036] (1) obtaining a lactate feed solution from a fermentation brothor other source;

[0037] (2) Modifying the lactate feed solution, preferably with a sourceof moderate or weak acid while maintaining a pH of at least 4, andpreferably within the range of 4-14; and

[0038] (3) Extracting the lactic acid with a water-immiscible weak ormoderate base, or mixture.

[0039] In the preferred applications described, the water-immiscibleweak or moderate base is a water-immiscible amine, typically an alkylamine and preferably a water-immiscible tertiary amine. Preferably alkylamines and most preferably water-immiscible trialkyl amines are used. Asexplained in the parent, the preferred water-immiscible amine will be atrialkyl amine containing at least a total of 18 carbon atoms.

[0040] In an alternate statement, typically when the extraction occurs,the aqueous phase is provided such that the lactate-containing componentor species present, at any given time throughout the extraction, is atleast 50% (molar equivalent) in the form of lactate, rather than thelactic acid. In general this is accomplished by appropriate control ofthe pH, and selection of the desirable acid for acidification of thesystem.

[0041] The preferred processes are conducted on lactate feed solutionsthat contain lactate values in a concentration of at least 3 mol. Whenthe processes concern recovery from a fermentation broth, generally thefermentation process is conducted such that the feed solution containsat least 5%, and typically 10 to 30%, by weight, lactate. The broth maybe concentrated, before extraction.

[0042] As also will be apparent from the parent disclosures, preferablythe moderate or weak acid, used to acidify, is an acid which is eitherreadily separated from the water-immiscible weak or moderate base, orwhich does not combine to any substantial extent with thewater-immiscible weak or moderate base under the conditions ofextraction. Carbon dioxide is an almost ideal acid for use in generationof the lactic acid, since its presence can so readily be controlledthrough control of its partial pressure, it can easily be removed fromthe solutions, it is relatively inexpensive, and it is such a weak acidthat the salt which is generated is also very suitable for effectiveneutralization of fermentation broth. It is foreseen that in someinstances the moderate or weak acid may comprise a salt of an acidhaving more than one proton; for example monosodium phosphate, providedthe pK_(a) for the remaining proton(s) is within the appropriate ranges.

[0043] In typical preferred applications, the acid which is used toacidify the multi-phase system is preferably a weaker acid (i.e. has ahigher pK_(a)) than lactic acid. A reason for this preference is thatthe corresponding salt of the added acid, which will form in the aqueousphase, will be a useful base in the fermentation broth. Also, when usedwith the preferred extractants, such as the water-immiscible amines, anadvantage results because of the extractant's preference for thestronger acid, i.e. lactic acid.

[0044] As indicated in the experiments, processes according to theinvention are characterized by relatively high recoveries of the lactatevalues in the lactate containing feed solution (for example, thefermentation broth). Recoveries greater than 90% are readily achieved,and typically the recovery is at about 95% or greater.

[0045] With respect to the weak or moderate base, to be used in theextraction, as indicated in the parent application, it has been foundthat trialkyl amines having a total of at least 18 carbon atoms, andpreferably from 24-42 carbon atoms, are most desired. Among the onesidentified in the parent application, tridodecylamine presently appearspreferred. However, in general it is believed that while tertiary aminesare preferred, substituted tertiary amines and in some instances evenprimary or secondary amines may also be used, provided they aresufficiently water-immiscible and perform as weak or moderate bases. Itis foreseen that, in many instances, primary amines may have sufficientwater solubility to be undesirable with respect to possiblecontamination of the aqueous phase. It is also foreseen, that in someinstances, primary or secondary amines may have too great a propensityto react with the lactic acid to form an amide, to be fully preferred.However, there is no theoretical reason why at least some secondary orprimary amines could not be used, under certain circumstances.

[0046] Although the general principles have been described with respectto conduct of the extraction with either a weak base or a moderate base(or mixture of both), in general it is believed that if a weak base isused alone, the extraction results will not be as great as preferredbecause the weak base typically will prefer the undissociated acid,which was the weaker acid used for acidulation. The lactic acid, whichis largely dissociated, is thus less effectively extracted by a weakbase (when the weaker acid is used). Thus it is foreseen that if a weakbase is to be used, it will generally be preferred to use it in amixture with a moderate base as well. A weak base is typically used as aco-solvent (or solvent) in the water-immiscible phase, with a moderatebase also present to facilitate extraction.

[0047] In the parent application, it was explained that while a varietyof extraction processes may be used, it was foreseen that countercurrentextraction processes would be preferred. In addition to providing forcountercurrent contacting of the two liquid phases, preferably theextraction unit used should provide for good removal and flow of thesolids formed during precipitation of the salt of the moderate to weakacid. In the preferred processes described, this would be thebicarbonate salt of the cation in the fermentation broth. Typically thecation will be Na⁺. In the most preferred embodiments, wherein thelactate feed treated and extracted has a greater than 3 mol. lactateconcentration and the extraction is conducted with a water-immiscibletrialkyl amine, the aqueous phase is relatively small in volume and, mayfor a slurry with the solid precipitate.

[0048] In the parent applications, solvents and/or diluent for thetrialkyl amines, i.e. solvents and/or diluents for the water-immisciblebases, were described. In general, the selection of solvent will becontrolled by a variety of factors such as boiling point, watersolubility, and ability to enhance the extraction efficiency of thewater-immiscible moderate base. However, in general when the lactic acidrecovery is from a lactate containing fermentation broth, it is desiredthat the extraction occur within as short a period of time of thegeneration of the fermentation broth as possible, for commercialefficiencies. Most fermentation broths are separated from thefermentation process at a temperature of about 50° C., in typicalcommercial operations. Thus, unless time is used in substantial cooling,or cooling equipment is used, the extraction will typically occur of anaqueous solution having a temperature of at last 35° C., usually about40-50° C.

[0049] If a back-extraction is used to recover the lactic acid from theorganic phase, preferably the aqueous phase used during theback-extraction is at a higher temperature than the aqueous phase fromwhich the lactic acid is separated in the first instance. This helpsincrease recovery efficiencies. Preferably the temperature of theback-extraction is as high as reasonably possible. Indeed, preferably itis conducted at a temperature of at least 100° C., and typically andpreferably at least 135° C.

[0050] When a solvent and/or diluent is present in the organic phase, aswill generally be preferred, the extraction is preferably conductedbelow the boiling point of the solvent and/or diluent. Thus, thetemperature of the desired aqueous phase will, in some instances,dictate the preferred solvent used. It has been found that a mixture ofparaffin (Isopar K, from Exxon) and octanol is a desirable solvent forthe organic phase, and the use of octanol allows extraction temperaturesup to about 140°-160° C. The mixture would preferably comprise about 50%trialkyl amine, 30% n-octanol and 20% non-aromatic paraffin, by weight.

[0051] It is noted that in some instances the solvent (or co-solvent) inthe water-immiscible phase will function as a weak base and thus becapable of some modest amount of extraction. This would be true, forexample, when the solvent is an alcohol or a ketone. However, alcoholsand ketones are generally such weak bases that their operation inextracting the lactic acid is more akin to a solvation process than amore tightly associated ion attraction with the lactic acid. Thus thepresence of at least some moderate base, such as the amine bases, isgenerally preferred. The presence of a polar solvent such as an alcoholor ketone can enhance the ability of a moderate base, such as awater-immiscible amine, to extract an acid, such as lactic acid.

[0052] If the organic phase is back-extracted, for final recovery of thelactic acid, in some instances it may be preferable to conduct theback-extraction under carbon dioxide pressure, in order to facilitatethe extraction. For example, if the primary extraction was conductedunder carbon dioxide pressure, the conduct of the back-extraction underpressure will prevent CO₂ release or the need to repressurize theescaping CO₂.

[0053] In preferred applications, the step of acidifying includesacidifying the aqueous phase from a fermentation process, to form sodiumbicarbonate in the aqueous phase; and, the process includes using thesodium bicarbonate to form sodium lactate in the first aqueous phase, inlater processing. This latter step may include forming sodium carbonatefrom the sodium bicarbonate and then putting the sodium carbonate in afermentation process.

DESCRIPTION OF THE DRAWING

[0054] The single FIGURE of the drawing is a block diagram representingthe preferred embodiment of the process according to the presentinvention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PROCESS

[0055] With reference to the drawing, lactic acid fermentation iscarried out in a fermentor 10 in which carbohydrates are fermented andconverted to lactic acid by the bacterial genus Lactobacillus and morespecifically by the microorganism Lactobacillus acidophilus. Becausemany organisms which are attractive in such a fermentation processcannot tolerate acidic conditions with a pH lower than about 3.8, theacids formed by this process must be at least partly neutralized tomaintain the pH above such level and more preferably above a pH of 4.5to allow the fermentation to continue. In accordance with the preferredprocess, a neutralizing agent such as the alkali metal, alkaline earthmetal or ammonium hydroxides, carbonates or bicarbonates are used forthis purpose. In the preferred process, sodium carbonate (Na₂CO₃) isadded to the fermentor 10 for this purpose, either via the recycle 12 asdiscussed below or along the path 13. Preferably, sufficient sodiumcarbonate or other alkaline substance is provided to the fermentor 10 tomaintain the pH of the fermentation broth at a pH above 5.0 andpreferably in the range of about 5.0 to 7.0, more preferably in therange of about 5.5 to 6.5 and most preferably in the range of about 6.0to 6.5. Other ingredients may also be used in the fermentation processwhich is well known in the art.

[0056] In the fermentor 10, the carbohydrate is converted to lactic acidwhich immediately is converted to a lactate form in the presence of theneutralizing agent. In the preferred process using sodium carbonate,sodium lactate [NaCH₃CH(OH)COO] is formed. A portion of the fermentationbroth or liquor is continuously or intermittently withdrawn from thefermentor 10 via the path 14 and exposed to a filtration andconcentration unit 15. The unit 15 functions to physically remove, viafiltration or ultrafiltration, biomass and other solids which can berecycled to the fermentor 10, if desired. The filtrate comprises anaqueous lactate solution which contains the lactate salt comprised ofthe lactate anion together with the cation of the neutralizing agent. Inthe preferred process, the filtrate is comprised principally of sodiumlactate. This solution, which commonly comprises between about 0.25% and50% by weight of sodium lactates may be concentrated by waterevaporation or other techniques to improve the overall lactic acidproduction efficiency. In the preferred process, the filtered lactatesolution is concentrated by water evaporation to about 40% to 70% byweight sodium lactate; however, such concentration is optional.

[0057] The sodium lactate solution exiting from the filtration andconcentration unit 15 comprises a lactate feed solution which is fedinto an extraction unit 18 along the path 16. The unit 18 is part of anextraction system which also includes the extractant regeneration unit22, the organic phase stream 21 and the extractant recycle stream 24.Within the unit 18, the lactate feed solution is combined with anextractant comprised of at least one water-immiscible trialkyl amine inthe presence of carbon dioxide, where the amine has a total of at least18 carbon atoms. Within the unit 18, two separate phases are formed: anorganic phase containing the extractant and extracted lactic acid and anaqueous or aqueous-slurry phase containing the carbonate or bicarbonatesalt of the cation of the neutralizing agent. In the preferred process,the aqueous phase contains sodium carbonate or bicarbonate. The unit 18may comprise any one of a variety of single or multi-stage pressureextraction units. In the preferred process, the unit 18 is a multi-stagecountercurrent extraction unit.

[0058] In the preferred process, the extraction system is initiallycharged with the trialkyl amine. The amine may be introduced by directlyadding it to the unit 18 or by adding it to the recycle stream 24through the amine make-up stream 17. During steady state operation,little if any additional trialkyl amine will be needed. To the extent itis, however, it can be added to the recycle stream 24 via the make-upstream 17.

[0059] Carbon dioxide may be added directly to the unit 18 underpressure via the page 19 a, to the organic recycle stream 24 underpressure via the path 19 c, or to the aqueous lactate stream 16 underpressure via the path 19 b. In the preferred process as illustrated, theorganic recycle stream 24 is preloaded with carbon dioxide by addingcarbon dioxide under pressure via the stream 19 c prior to the unit 18.In any case, the carbon dioxide within the unit 18 should preferably bemaintained at a partial pressure of at least about 50 psig, morepreferably at a partial pressure of at least 75 psig and most preferablybetween about 150-500 psig.

[0060] Although it is believed that some extraction of lactic acid froma lactate solution is possible with any water-immiscible trialkyl aminein the presence of carbon dioxide, the particular degree of extractionwill vary with the amine utilized and the carbon dioxide pressure. Thedegree of extraction can also be enhanced or otherwise affected byvarious solvents as described below and as known in the art. The degreeof extraction will generally be dependent on the partition coefficientand the number of stages used in the extraction process. As used herein,the partition coefficient is the mass concentration of lactic anion inthe organic phase divided by the mass concentration of lactate expressedas lactic acid equivalent in the aqueous phase. Usually, for aparticular system, the partition coefficient, within limits, will varydirectly with the carbon dioxide pressure. As the partition coefficientincreases, the number of stages needed to achieve a particular degree ofextraction will decrease. Carbon dioxide and amine composition shouldpreferably be sufficiently high to avoid excessive extractant phasenecessary to extract the acid.

[0061] The trialkyl amines which are useful in the process of thepresent invention are those which are water-immiscible and relativelyweak. Specifically, these are the trialkyl amines having a total of atleast 18 carbon atoms and preferably about 24 to 42 carbon atoms. Thepractical lower limit of the number of carbon atoms is limited by theincreasing water solubility of the smaller trialkyl amines or theirsalts. The water immiscibility of the trialkyl amines with 18 or morecarbon atoms is well known in the art. The practical upper limit of thenumber of amine carbon atoms is determined by the molar concentration ofamine obtainable in the organic phase. Specifically, the extractionability of the trialkyl amines is dependent on the molar concentrationof the amine component. Thus, as the molecular weight of the amineincreases, the molar concentration of the amine component (or a pureamine solution) will decrease. The trialkyl amine should also besufficiently strong to extract the lactic acid from the aqueous lactatefeed, but sufficiently weak to enable water to back extract the lacticacid from the organic phase. Typical examples of such amines which meetthe above requirements, are readily available and are useful in theprocess of the present invention are one or more of trihexylamine,trihexylamine, trioctylamine, triisooctylamine, tricaprylylamine,tridodecylamine and mixtures thereof.

[0062] The particular ratios of lactate feed solution and trialkyl aminephase which are fed to the unit 18 along the paths 16 and 24,respectively, will depend on a variety of factors including theconcentration of the sodium lactate and the concentration of the amine.Preferably, the introduction of these materials should be such as toresult in a substantially complete extraction of lactic acid from thelactate solution with the number of stages utilized. More preferably,the feed ration of amine phase to lactate solution should be about 40:1to 1:2 and most preferably about 15:1 to 1:1 by weight.

[0063] The trialkyl amine provided to the unit 18 may be introduced in asubstantially pure or a diluted form. Because many of the aminesapplicable to the present process and their salts are relativelyviscous, it is preferably to introduce such amines with a solvent. Ingeneral, any composition which is miscible with the subject amines andtheir salts within the range of compositions used and which isreactively inert relative to the system components may be used in thepresent process. These solvents may be used to control viscosity,enhance extraction or stabilize the organic phase in a manner generallyknown in the art. Typical examples of solvents which can be used in thepresent process include liquid hydrocarbons such as kerosene or mineraloil, alkanols such as isopropanol, n-butanol and n-octanol and variousketones such as methyl-isobutyl ketone (MiBK) and nonanone, amongothers. The extractant used in the process of the present invention maycomprise 100% of the trialkyl amine. A more preferred extractant,however, comprises up to about 70% by weight of a solvent or shouldcomprise about 30%-95% by weight of the amine and about 5%-70% by weightof the solvent.

[0064] Following extraction within the unit 18, a lactic acid-richorganic phase comprised of lactic acid and the extractant is withdrawnalong the path 21 and an aqueous phase or slurry comprises principallyof carbonate and/or bicarbonate is withdrawn along the path 20. As usedin the description of the preferred embodiment, the term carbonate orbicarbonate refers to the carbonate or bicarbonate salt of the cation ofthe substance used to neutralize the fermentation. Within the aqueousphase or slurry of the preferred embodiment, the predominant carbonateor bicarbonate is sodium bicarbonate which exists principally as sodiumbicarbonate crystals. These are separated from the aqueous raffinate inthe solid-liquid separation unit 25. The unit 25 can comprise variousfiltration, centrifugation or other solid-liquid separation means knownin the art. Preferably, however, the sodium bicarbonate crystals areseparated by filtration. The aqueous filtrate which in the preferredprocess is substantially free of lactate may be removed as a componentof animal feed or as waste along the path 26. It is also possible, ifdesired, to recycle all or part of the filtrate back into the systemthrough the streams 11, 14 or 16.

[0065] The separated sodium bicarbonate is then directed along the path29 to a conversion and purification unit 30 for conversion of the sodiumbicarbonate to sodium carbonate. Means are known in the art foraccomplishing this conversion. In the preferred process, however, thesodium bicarbonate crystals are decomposed in boiling water to producecarbon dioxide and dissolved sodium carbonate. The solution of sodiumcarbonate is then purified by active carbon treatment and recycled alongthe path 12 as an alkaline or neutralization component in thefermentation process. The released carbon dioxide can also be reused, ifdesired. Since the preferred process utilizes sodium carbonate as theneutralizing component in the fermentation process, the aqueous phaseafter fermentation (stream 14) is comprised of sodium lactate. It iscontemplated that other alkali metals, alkaline earth metals or ammoniumhydroxides, carbonates or bicarbonates may also be used as theneutralizing agent, in which event the cations in the aqueous phasewould be altered accordingly.

[0066] The lactic acid-rich organic phase is withdrawn from the unit 18along the path 21. In the preferred process, the organic phase isdecompressed in the flash unit 32 as it leaves the unit 18. This resultsin the release of a majority of the dissolved carbon dioxide via thestream 33 which may be recycled to streams 19 a, 19 b or 19 c, ifdesired. This organic phase is made up principally of lactic acid andthe extractant. Lactic acid is separated or recovered from this phase inthe extraction unit 22, leaving a lactic acid-lean or depleted organicphase which is preferably recycled back to the extraction unit 18 alongthe path 24 in the form of regenerated extractant. As described above,carbon dioxide may also added to the recycle stream 24 via the path 19 cto load the amine prior to the unit 18. The lactic acid solution isremoved from the unit 22 as product via the stream 28.

[0067] In the preferred process, the unit 22 is a liquid/liquidextraction unit within which the lactic acid-rich organic phase is backextracted with water introduced along the path 23. Because of therelatively weak amine being used in the primary extraction process andbecause the amine is water-immiscible, the water is able to extract thelactic acid from the amine to form an aqueous solution of lactic acid ofacceptable concentration.

[0068] In the case where the trialkyl amine is diluted with anappropriate solvent, such solvent becomes a part of the organic phasewithdrawn from the unit 18 along the pat 21. Some solvents, such asalkanols and ketones, modify and enhance the lactic acid uptake into theorganic phase. It is preferable to remove such solvents prior to theback extraction with water in the unit 22. This separation of thesolvent from the lactic acid-rich organic phase can be accomplished byvarious separation techniques known in the art. Preferably, whenpossible, the separation is by azeotropic steam distillation within theunit 27. The removed solvent from the separation unit 27 may then berecycled along the pat 31 for regeneration of the extractant and use inthe primary extraction, if desire.

[0069] The lactic acid can also be separated or recovered from theorganic phase by vaporization or distillation of the lactic acid.Removal by distillation should preferably be performed at reducedpressure and elevated temperature conditions. Most preferably, theseparation should be accomplished at pressures of from about 0.2 to 100mm Hg and at temperatures from about 80° C. to about 240° C. If thisdistillation option is employed, the trialkyl amine should have a totalof at least 24 carbon atoms, or be sufficiently nonvolatile to allowlactic acid fractionation from the amine by vacuum distillation.

[0070] The vaporization conditions will also remove alkanols or ketones,if present, as well as other solvent components more volatile than thetrialkyl amine. These can be separately recovered and may be returned tothe depleted extractant before it is cycled back to the extraction step.The vapor of lactic acid thus formed may also be directly fed, ifdesired, to an esterification process for reaction and furtherpurification.

[0071] With the above process, lactic acid can be separated and/orremoved from a lactate fermentation broth. Under optimal conditions,such separation and/or recovery can approach total recovery of thelactic acid, greater than 95% of that produced by fermentation. Of equalor greater importance is the ability of this recovery to be accomplishedwith the generation of minimal, if any, waste salt and undercircumstances where substantially all of the extraction, conversion andother components used in the process can be recycled for reuse withinthe process. Still further, the process is significantly less energyintensive than competing processes and results in minimal, if any, plantemissions.

[0072] The preferred process has been described with respect toproducing lactic acid from a lactate solution formed via a fermentationprocess. The present process is, however, application to the separationand/or recovery of lactic acid from a lactate solution regardless of itsorigin. For example, polylactide is a biodegradable polymer producedfrom lactic acid. Polylactide can be recycled by hydrolysis of thepolymer to yield a lactate salt. The present process can then be used torecover lactic acid from the lactate salt for reuse in formation of thepolylactide polymer.

[0073] Further details of the present process are shown and described inthe following specific examples.

EXAMPLE 1

[0074] A lactate fermentation broth containing 10% by weight ofNaCH₃CH(OH)COO (sodium lactate) was withdrawn from a fermentor in whichpure carbohydrates were fermented by Lactobacillus delbrueckii in thepresence of sodium carbonate in order to maintain a pH of 5.5, all asknown in the art. The biomass and other solids were removed from thefermentation broth by filtration through ultrafiltration membranes andthen concentrated by water evaporation to 50% by weight sodium lactate.

[0075] 150 g/min of this sodium lactate solution were fed to a 5-statemixer-settler battery counter-currently to 1050 g/min of regeneratedextractant comprising 48% by weight tricaprylylamine (Alamine 336™produced by Henkel), 20% by weight n-butanol and 32% by weightaromatic-free kerosene. The extraction system was kept at ambienttemperature and a 240 psig CO₂ atmosphere was maintained therein. Sodiumbicarbonate crystals formed in the aqueous phase as of the secondmixer-settler.

[0076] The aqueous phase was withdrawn and sodium bicarbonate wasfiltered off from the aqueous raffinate which was practically free oflactate values. The sodium bicarbonate crystals were decomposed inboiling water to CO₂ and to dissolved sodium carbonate. This solutionwas purified by active carbon treatment to a form suitable for use as abase in the fermentation.

[0077] The organic phase withdrawn from the last stage of themixer-settler battery contained 0.65 mole lactic acid per kg. CO₂ wasallowed to escape from the organic phase, following which the butanolwas separated and removed by azeotropic steam distillation. Theremaining organic phase was back-extracted with hot water to form alactic acid-depleted organic phase and an aqueous solution of the lacticacid. The separated butanol was reintroduced into the back-extractedorganic phase to regenerate the extractant while the aqueous phase wasconcentrated and fed to final purification. Removal of lactic acid fromthe lactate feed solution was about 95%.

EXAMPLE 2

[0078] Aqueous solutions of sodium lactate were equilibrated in apressure vessel with various extractants under a 220 psig CO₂atmosphere. Contact temperature was 20° C. The initial pH of the aqueousphase and equilibrium data are summarized in the Table below. TABLE 1Equilibrium data sodium lactic Initial lactate acid Extractant aqueousaqueous (wt organic composition pH %) (wt %) n-butanol 5.5 6.2 1.05 80%TDA + 20% i-PrOH 5.5 29.2 10.6 80% TDA + 20% hexane 5.5 39.8 2.9 67%TCA + 33% n-OctOH 10.9 50.3 14.3 70% TCA + 30% n-BuOH 10.9 50.3 15.0 48%TCA + 20% n-BuOH + 10.7 46.9 9.3 32% kerosene

EXAMPLE 3

[0079] An extractant mixture comprised of 80% by weight tridodecylamine(Alamine 304-1) produced by Henkel) and 20% by weight n-butanol, wascontacted with 30% by weight aqueous lactic acid (Purac) in sufficientquantity to produce a loading of 6.9% by weight lactic acid in theorganic phase. 230 g of this material was added to a stirred roundbottom flask connected to a distillation apparatus, condenser, andcontrolled vacuum system. The solution was heated to 219° C. at apressure of 2 mm Hg. Initial condensate fractions included butanol andwater. A later fraction showed recovery of 97% by weight of the originallactic acid. The residual extractant contained 0.2% by weight lacticacid. The composition of the pooled fractions containing the acid was98.3% by weight aqueous lactic acid and 1.7% by weight of its oligomers.The depleted extractant was replenished with butanol and cycled back foranother extraction. Five such cycles were rn on one batch withoutsignificant loss of extractant performance.

EXAMPLE 4

[0080] Various experiments were conducted in a Parr pressure reactor forthe purpose of showing the applicability of the process of the presentinvention to a broad range of CO₂ pressures, to a variety of solventsand to representative samples of trialkyl water-immiscible amines with atotal carbon content of at least 18.

[0081] The apparatus comprised a Parr pressure reactor with fouragitators, gas inlet and outlet ports, aqueous and organic sample portsand a pressure gage. The procedure involved adding the aqueous and thepre-mixed organic solutions to the Parr reactor. Except for ExperimentNos. 7, 12, 18 and 21 below in which the ratio of organic to aqueous was1:3, the ratio of organic to aqueous in all experiments was 1:1. Theaqueous solution was comprised of sodium lactate (NaLa), calcium lactate(CaLa) or potassium lactate (KLa). The NaLa solutions comprised about20%-40% by weight of the lactate, the KLa solution comprised about 20%by weight of the lactate, while the CaLa solution comprised about 6% byweight of the lactate. Further, in Experiment Nos. 7-10, 12-18 and20-24, 10% by weight sodium bicarbonate was added for the purpose ofsaturating the solution. The organic solution comprised a trialkyl amineor a mixture of a trialkyl amine and one or more solvents.

[0082] The Parr pressure reactor was then assembled and a slow flow ofCO₂ was introduced for about 5 minutes to purge the air in the reactor.The CO₂ pressure was then adjusted to the desired level. It should benoted that the pressures identified in the table below are gagepressures. Thus, a CO₂ level of O psig as indicated in Experiment Nos. 1and 13 reflect a CO₂ partial pressure of 14.7 psi. The CO₂ pressure wasmaintained at the selected level, with a slow bleed of CO₂ (about 100ml/min) bubbling through the contents, and the contents in the reactorwere agitated for two hours. The CO₂ inlet and outlet ports were thensealed and agitation continued for 10 more minutes, at which timeagitation was terminated and the contents were allowed to settle for 30minutes. Samples of both the organic and aqueous phases were collectedthrough the organic and aqueous sample ports, after which the aboveprocedure repeated for a different CO₂ pressure. All experiments wererun at 25° C.

[0083] All organic samples were analyzed with NaOH to a phenolphthaleinendpoint to determine concentration of lactic acid in the organic phase.All aqueous samples were analyzed by HPLC to determine lactic acidequivalent in the aqueous phase. The partition coefficient (K) was thencalculated by dividing the concentration of lactic acid in the organicphase by the concentration of lactate, expressed by lactic acidequivalent, in the aqueous phase.

[0084] The table below reflects data from selected experiments conductedin accordance with the above procedure in which the amines and solventsare identified as follows. All percentages are by weight unlessotherwise specified. Amines Solvents A1 = trihexylamine CS1 = n-octanolA2 = trioctylamine CS2 = n-butanol A3 = triisooctylamine CS3 = nonanoneA4 = tricaprylylamine CS4 = Isopar K, Exxon A5 = tridodecylamine

[0085] TABLE 2 Final A- CO₂ K Aq. que- Press. C (org)/ Wt % No Organicous (psig) C (aq) Lac.  1 A4 (48%), CS1 (30%), CS4 (22%) CaLa  0 0.0682.21  2 A4 (48%), CS1 (30%), CS4 (22%) CaLa 150 0.388 2.06  3 A4 (48%),CS1 (30%), CS4 (22%) CaLa 220 0.553 1.99  4 A4 (48%), CS1 (30%), CS4(22%) CaLa 300 0.622 1.93  5 A4 (48%), CS1 (30%), CS4 (22%) KLa  750.112 18.5  6 A4 (48%), CS1 (30%), CS4 (22%) KLa 500 0.198 17.5  7 A4(48%), CS1 (30%), CS4 (22%) NaLa  75 0.047 19.9 (sat)  8 A4 (48%), CS1(40%), CS4 (12%) NaLa 150 0.093 38.3 (sat)  9 A4 (48%), CS1 (40%), CS4(12%) NaLa 200 0.155 38.0 (sat) 10 A4 (48%), CS1 (40%), CS4 (12%) NaLa300 0.199 37.5 (sat) 11 A4 (48%), CS2 (35%), CS4 (17%) NaLa 220 0.25529.8 12 A1 (33%), CS1 (30%), CS4 (37%) NaLa 240 0.123 20.1 (sat) 13 A2(43%), CS1 (30%), CS4 (27%) NaLa  0 0.038 21.9 (sat) 14 A2 (43%), CS1(30%), CS4 (27%) NaLa  75 0.064 21.6 (sat) 15 A2 (43%), CS1 (30%), CS4(27%) NaLa 240 0.119 20.7 (sat) 16 A4 (48%), CS3 (52%) NaLa 240 0.06921.6 (sat) 17 A3 (43%), CS1 (30%), CS4 (27%) NaLa  75 0.038 20.3 (sat)18 A3 (43%), CS1 (30%), CS4 (27%) NaLa 240 0.097 20.0 (sat) 19 A5 (48%),CS2 (20%), CS4 (32%) NaLa 220 0.266 34.4 20 A4 (100%) NaLa  75 0.02021.4 (sat) 21 A4 (100%) NaLa 240 0.047 21.4 (sat) 22 A4 (43%), CS1(30%), CS4 (27%) NaLa 500 0.208 19.8 (sat) 23 A4 (48%), CS3 (52%) NaLa500 0.102 21.3 (sat) 24 A4 (48%), CS4 (52%) NaLa 500 0.020 21.8 (sat)

EXAMPLE 5 Extraction of Lactate Feeds at Various Conditions

[0086] Sodium and potassium lactate solutions were preparedstoichiometrically from lactic acid solutions. A304 is Alamine 304,Trilaurylamine from Henkel Corp. The concentration was adjusted to 1mol/kg by diluting with Parasol, a non-aromatic kerosine solvent,boiling range 210-275° C., from Paz Company of Israel. The alcohols weresupplied by Merck or Frutarom of Israel.

[0087] The pressure experiments were carried out in a Parr bench-topmini reactor (Serial 4560, 300 ml autoclave). CO₂ was supplied via acylinder. The control panel regulated the heating and measured internalpressure in psi. Stirrer speed was 600 rpm. At each pressure level astable pressure was held for 15-20 minutes to ensure completeequilibrium and then phase separation for a further 15-20 minutes.Samples were taken (for Table 3 experiments) from the organic phase,which were usually clear. Free CO₂ volume was measured by waterdisplacement. Lactic acid concentration was determined by titration with0.1 N NaOH and phenolphthalein indicator after warming the sample forremoval of extracted CO₂. TABLE 3 CO₂/ Loading gm H₂O Org. Org. PhaseOrg. Conversion/ Initial Pressure Temp Phase Phase Ratio PhaseExtraction Exp Amine Conc. Modif. Conc. Aqueous Conc. psi ° C. mol/kg ccorg/ag % % 19 A304 1 mol/kg 2-BuOH 20% NaLa 40% 87 14° 0.27 3 145 0.43212 0.539 20 None 2-BuOH 100%  NaLa 40% 85 14° 0.114 3 146 0.1 218 0.11321 A304 1 mol/kg 2-BuOH 20% NaLa  5% 88 14° 0.053 3 146 0.069 217 0.08222 A304 1 mol/kg 2-BuOH 20% KLa    8.30% 87 13° 0.021 15.3 3 147 0.01922.9 218 0.027 36.7 23 A304 1 mol/kg 2-BuOH 20% NaLa 20% 218 18° 0.228 324 A304 1 mol/kg 2-BuOH 20% NaLa 10% 218 14° 0.1 35.9 3 25 A304 1 mol/kgi-BuOH 30% NaLa 56% 238 18° 0.923 73.9 3 57.4 26 A304 1 mol/kg i-BuOH30% NaLa 56% 240 18° 0.785 43.4 5 1.41 82.1 27 A304 1 mol/kg i-BuOH 30%KLa 59% 238 18° 0.657 39.9 5 1.26 71.9 28 A304 1 mol/kg i-PrOH 20% NaLa56% 244  0° 0.831 90.0 5 1.46 86.1 238 16° 0.781 40.1 5 1.01 82.0 23750° 0.464 62.5 5 1.44 47.7 29 A304 1 mol/kg EtOH 15% NaLa 56% 239  0°0.655 73.6 5 1.4 30 A304 1 mol/kg i-PrOH 20% NaLa 56% 237  0° 0.707 62.75 67.7 238 17° 0.859 44.0 5 83.3 31 A304 1 mol/kg EtOH 15% NaLa   51.80%238 20° 0.722 48.2 5 75.5 32 None EtOH 100%  NaLa   51.80% 236 21° 0.14375.1 5 33 A304 1 mol/kg n-Octanol 20% NaLa   51.80% 240 20° 0.526 34.1 775.9 34 A304 1 mol/kg i-PrOH 30% NaLa   51.80% 241 18° 0.848 43.4 5 24040° 0.803 31.82 5 238 59° 0.593 31.36 5

EXAMPLE 6 Extraction With Variations in Extractant and pH

[0088] Materials

[0089] A304

[0090] Alamine 304, Trilauryl Amine. Henkel Corp. commercial product

[0091] A336

[0092] Alamine 336, Tri-Caprylyl Amine (C₈-C₁₀). Henkel Corp. commercialproduct

[0093] DEHPA

[0094] Di-(2-ethylhexyl) Phosphate Sigma, Anhydrous reagent

[0095] i-AmOH

[0096] Merck A.R.

[0097] 1-BuOH

[0098] Frutarom A.R.

[0099] 2-BuOH

[0100] Frutarom A.R.

[0101] i BuOH

[0102] Merck A.R.

[0103] n-Octanol

[0104] Merck A.R.

[0105] Lactic acid (LaH)

[0106] Merck A.R., 90% solution

[0107] MIBK

[0108] Methylisobutylketone Frutarom C.P.

[0109] MTCA

[0110] Aliquat 336, Methyltricapryl amine chloride, Henkel Corp.

[0111] n-Butyl Acetate

[0112] BDH GPR

[0113] Parasol

[0114] Kerosine solvent <1% aromatics, boiling range 210-275° C., PazCompany

[0115] Primene JM-T

[0116] 5-Alkyl Primary Amine (a primary aliphatic amine with highlybranched alkyl chains, nitrogen is bonded directly to a tertiary carbon,R₁C(R₂) (R₃)NH₂) ROHM & HAAS, commercial product

[0117] TBP

[0118] Tri-Butyl Phosphate Riedel-de Haen 99% solution

[0119] TOPO

[0120] Trioctylphosphine oxide, Sigma 90% tech

[0121] Xylene

[0122] Frutarom A.R.

[0123] Methods

[0124] Distribution curves were determined by limiting conditionexperiments carried out in 100 ml erhlenmeyers. Analysis of clearorganic phase for loading of lactic acid was by direct titration withNaOH 0.1N in isopropanol using phenolphthalein as indicator. The aqueousphases were similarly analyzed in H₂O.

[0125] Sodium lactate solutions at the various concentrations and pH'swere prepared stoichiometrically from lactic acid solutions thenadjusted to the required pH. The extraction curves for the variousextractants and solvents were made at limiting conditions and analysisas above on clear organic phase.

[0126] For all experiments the lactic acid was diluted from the originalbottled solution (90%) and hydrolysed by refluxing for 4-6 hours. TABLE4 pH = pH = pH = pH = Organic Phase 2-2.3 3-3.1 4-4.1 ph = 4.9 6-6.3 pH= 6.9 pH = 8 1 mol/kg A304 in Parasol + 1.26 1.07 0.57 0.057 0.011 10%i-BuOH 1 mol/kg A304 in Parasol + 1.48 1.48 1.07 0.29 0.011 30% i-BuOH 1mol/kg A304 in Parasol + 1.15 0.49 0.031 0.000 20% n-octanol 1 mol/kgJMT in Parasol + 1.05 0.97 0.89 0.087 0.028 20% n-octanol 1 mol/kg JMTin n-octanol 1.15 0.82 0.089 0.048 1-butanol 0.87 0.25 0.000 0.000 isoamyl alcohol 0.60 0.22 0.000 0.000 TBP 0.74 0.28 0.016 0.000 1 mol/kgTOPO in xylene 0.75 0.68 0.56 0.21 0.043 n-butyl acetate 0.10 0.0320.000 0.000 A304:Oleic acid 0.5 mol/kg 0.35 0.100 0.000 0.000 336:DEHPA0.5 mol/kg 0.39 0.33 0.26 0.082 0.000 MTCA:DEHPA 0.5 mol/kg 0.35 0.240.036 0.030 MTCA 1 mol/kg + 20% octanol 0.59 0.31 0.019 0.000

A Proposed Commercial Process

[0127] In the application of the disclosed invention, and based onlaboratory and pilot data, a commercial process is foreseen to have thefollowing preferred configuration and conditions. The process will bedescribed with reference to numerals used in the FIGURE.

[0128] Lactate fermentation by Lactobacillus would be conducted with asubstrate (11) of dextrose, salts, and nutrients, controlled at pH 5 to7 by addition of recycled sodium carbonate (12) in standard anaerobicfermentators (10). CO₂ evolved during fermentation would be recoveredfor reuse in the extraction. After fermentation is complete, thefermentation liquor (14) would be filtered (15) to remove biomass,carbon treated, and evaporated (15) to 40-70 wt % NaLa. The cells arediscarded after the process.

[0129] The concentrated broth (16) would be pumped into the extractionunit and contacted for liquid-liquid extraction at about 40° C. and 300psi in countercurrent flow with the extractant. The regeneratedextractant (24) would enter the opposite end of the extraction unit. Theextractant to broth flows would be maintained at flow ratios of about 10to 1. The extractant would be composed of about 50 wt % tricaprylylamine, 30 wt % n-octanol, and 20 wt % nonaromatic paraffin with nominalboiling range of 180-200° C.

[0130] The extraction unit (18) can be any one of many industriallyproven differential or discrete countercurrent extractors whichinherently or through modification are able to handle the solid slurryof sodium bicarbonate that is formed during the extraction, and shouldbe of a type constructed to operate under pressures up to 500 psig andbe designed to give about five equivalent equilibrium states. Three suchextractors are:

[0131] 1. Discrete stage mixer settlers, each mixer and settler stageconstructed as pressure vessels, and each settler constructed to allowconveying settled solids along the bottom to the discharge pipe, whichis also designed to avoid plugging by solids.

[0132] 2. Raining bucket contactors, also known as Graesser extractors,constructed as a pressure vessel, with modified internals to allow thesmooth conveyance of a viscous slurry in the aqueous phase.

[0133] 3. Agitated or stirred column extractors, constructed as apressure vessel and with modified internals to allow the smooth rainingof the slurry. Pulsing of flow-or agitation of the column internals ispreferred to prevent buildup of solids in the process.

[0134] The extractor (18) should be operated such that at least about90% of the incoming lactate values are converted to lactic acid and areextracted into the extractant in one pass. Remaining in the depletedbroth, or raffinate, will be lactic acid, sodium bicarbonate,unfermented sugars, and other broth impurities. Sodium bicarbonate willbe produced in amounts stoichiometrically equivalent to the moles oflactate converted. The final slurry phase raffinate (20) will have up to50 wt % solid sodium bicarbonate after extraction.

[0135] Raffinate exiting the extractor (20) will preferably be flashedto atmospheric pressure, with the liberated CO₂ being captured forreuse. The raffinate solids (29) would be removed by centrifugation orfiltration (25), washed with water and would be used to neutralizefermentation. In this way, sodium values are recycled. The aqueousraffinate (26) containing approximately 5-10% of the sodium values, plusall other impurities and lactate values of the raffinate (20), would beused in animal feed formulations. Sodium losses in raffinate should bemade up (13).

[0136] Loaded extract from the extractor (18), with lactic acid contentof greater than about 0.5 mol lactic acid/kg, would be flashed (32) toremove Co₂ (33), and the resultant stream (21) would be back-extractedin about five stages of an extractor train (22) in countercurrentcontact with water (23). This extractant regeneration is operated atabout 140° C. and 100 psig, with water to extractant flow ratios ofabout 2:1. Extractant regeneration can be carried out in one of manyindustrially available extraction devices, including mixer settlers,columns, or centrifugal extractors. The equipment must be designed tohandle the elevated temperatures and pressures.

[0137] The regenerated extractant (24) with a lactic acid content ofless than about 0.1 mol lactic acid/kg, would be saturated with CO₂ at300 psig (19 c), and added back to the extractor.

[0138] The resultant lactic acid stream (28), with a concentration of15-20 wt %, would be treated with activated carbon, cation exchange, andevaporation, to produce heat stable lactic acid.

[0139] Carbon dioxide collected from flashing of raffinate (20) andloaded extractant (33), as well as from fermentation (10) and sodiumbicarbonate decomposition (30) (if used), would be collected, purified,and recompressed for reuse in extraction (19 c).

[0140] By the above description, it can be seen that sodium recyclesfrom fermentation to extraction and back to fermentation. Extractant isrecycled from extraction to extractant regeneration. Carbon dioxide isrecycled from fermentation, bicarbonate decomposition, and flashing toextractant saturation. These three recycle schemes can provide forefficient and cost effective production of lactic acid while reducingthe environmental impact of the production process.

What is claimed is:
 1. A process for the recovery of lactic acid; saidprocess including the steps of: (a) forming a multi-phase systemincluding at least a first aqueous phase and a second, water-immiscible,liquid phase; (i) said first aqueous phase having a pH of 4 to 14 andincluding lactate salt in solution; (ii) said second water-immisciblephase including an extractant capable of forming a water-immisciblelactate salt, with a lactate-containing component; (b) extracting saidfirst aqueous phase with said second water-immiscible phase byextracting the lactate-containing component by forming awater-immiscible lactate salt with the extractant; (i) said step ofextracting including a step of acidifying at least one of said firstaqueous phase and said second water-immiscible phase, by addition ofacid thereto, without providing said first aqueous phase with a pH below4; (c) separating a resulting water-immiscible phase from a resultingaqueous phase after said step of extracting; and (d) generating lacticacid from the lactate salt of the extractant.
 2. A process according toclaim 1 wherein: (a) said step of forming a multi-phase system includesproviding a first aqueous phase having a temperature of 35°-50° C.,while said step of extracting is conducted.
 3. A process according toclaim 2 wherein: (a) said step of generating lactic acid from thelactate salt of the extractant includes a step of back-extracting thewater-immiscible phase, resulting from said step of separating, withwater.
 4. A process according to claim 3 wherein: (a) said step ofback-extracting includes back-extracting with water at a temperature ofat least 100° C.
 5. A process according to claim 1 wherein: (a) saidstep of extracting comprises extracting at least 90% of alllactate-containing component initially present in the first aqueousphase, into said water-immiscible phase.
 6. A process according to claim1 wherein: (a) said step of extracting comprises extracting with anextractant containing base selected from the group consisting of:moderate bases; and, mixtures of moderate bases and weak bases.
 7. Aprocess according to claim 1 wherein: (a) said step of extractingcomprises extracting with an extractant containing an amine base.
 8. Aprocess according to claim 7 wherein: (a) said step of extracting withan extractant containing an amine base comprises extracting with anextractant containing a tertiary amine.
 9. A process according to claim2 wherein: (a) said step of acidifying comprises acidifying with a weakacid.
 10. A process according to claim 9 wherein: (a) said step ofacidifying comprises acidifying with CO₂.
 11. A process according toclaim 10 wherein: (a) said step of acidifying comprises adding CO₂ tosaid second water-immiscible phase prior to said step of forming amulti-phase system.
 12. A process according to claim 10 wherein: (a)said step of acidifying comprises adding CO₂ to said first aqueous,prior to said step of forming a multi-phase system.
 13. A processaccording to claim 10 wherein: (a) said step of acidifying with CO₂comprises adding at least one molar equivalent of CO₂ for each mole oflactate salt initially in the solution.
 14. A process according to claim10 wherein: (a) said step of acidifying with CO₂ comprises providing apartial pressure of CO₂, in the multi-phase system, of at least 50 psig.15. A process according to claim 10 wherein: (a) said step of acidifyingwith CO₂ comprises providing a partial pressure of O₂, in themulti-phase system, of at least 300 psig.
 16. A process according toclaim 1 wherein: (a) said step of acidifying comprises acidifying with aweak acid; (b) said step of extracting comprises extracting with anextractant that contains base selected from the group consisting of:moderate bases; and, mixtures of weak bases and moderate bases; and, (c)said first aqueous phase is derived from a lactate fermentation broth.17. A process according to claim 16 wherein: (a) said first aqueousphase includes sodium lactate; (b) said step of acidifying with a weakacid comprises acidifying with CO₂; and, (c) said step of extractingcomprises extracting with a trialkyl amine.
 18. A process according toclaim 17 wherein: (a) said step of acidifying includes forming sodiumbicarbonate; and, (b) said process includes a step of recovering thesodium bicarbonate, and using it to generate sodium lactate in the firstaqueous phase, in further processing.
 19. A process according to claim18 wherein: (a) said step of using sodium bicarbonate to generate sodiumlactate comprises a step of forming sodium carbonate from the sodiumbicarbonate.
 20. A process for recovery of lactic acid; said processincluding the steps of: (a) forming a multi-phase system including afirst aqueous phase and a second water-immiscible phase; (i) said firstaqueous phase including lactate salt in solution; (ii) said secondwater-immiscible phase including lactic acid extractant; (b) extractingsaid first aqueous phase with said water-immiscible phase; (i) said stepof extractant including a step of acidifying at least one of said firstaqueous phase by addition of an acid thereto, and said water-immisciblephase under conditions wherein, at any given point, at least 50% oflactate-containing component in the aqueous phase remains in the lactatesalt form; and, (ii) said step of extracting comprises removing at least90% of lactate-containing component, as lactic acid, from said firstaqueous phase and into said water-immiscible phase; (c) separating aresulting water-immiscible phase from a resulting aqueous phase aftersaid step of extracting; and (d) generating lactic acid from the lactatesalt of the extractant.